Forced and Free Convection Heat Transfer Coefficients - Lab Report Example

FORCED AND FREE CONVECTION HEAT TRANSFER COEFFICIENTS Summary The convection laboratory experiment was separated into two parts, forced and natural or the free convection method. A flat aluminum plate or disk was attached inside an air duct. There was constant supply of 1500 watts to the plate or disk; while the air was being run over the duct at specific forced velocity, temperature was recorded at each second rage. Measurements were recorded for the steady state temperatures of the air and of the surface of the disk. The laboratory experiment was directed at two main objectives. The experiment measured heat transfer coefficients using the lumped mass approximations. The second was to analyse and examine uncertainties involved in the measurement. It is evident from the experiment that the convection heat transfer coefficient proportionally increased, in a linear manner, with increased in the air velocity for free convection, and over the range values of forced convection. Introduction In everyday life itself and in the field of chemical engineering, convection is one of the most significant forms of heat transfer. These ranges from running the 150,000 lb/hr of super-hot water via heat exchanger, to simple fan turning; the uses and principles of convection are the same. The forced convection is significantly utilized in designing heat exchangers. Forced convection is used in in order to increase the input temperature that leads to faster rate of reactions, or decreasing the temperatures of waste, or recovering heat that would have been wasted. For example, the paper industry utilizes heat exchangers in preheating their milling water prior to being send to the separator, in order to cool waste water prior to settling water treatment section; the principle permits the plant in recovering the heat that would have been wasted, therefore channeled in various parts of the plant for usage. Free convection, majorly, influences electronics board, transmission lines and pipe heat transfer. The most relevant utilization of the free convection is in the electronic components cooling; the performance life expectancy and reliability of electronic equipment is inversely related to the component’s temperature; reduction in temperature leads to an exponential increase in life expectancy or reliability of a device. Computer engineers, through studying free convection, may locate or positron heat sinks for cooling electronic components. Objectives 1. To measure heat transfer coefficients using the lumped mass approximation. 2. To examine uncertainties involved in the measurement. Theory Convection heat transfer is due to the transfer of energy among or between fluids at one temperature over solid surfaces at different temperature. Figure 1: Convection over a flat plate. The (meters/sec) is the air velocity, T∞ (°C) air temperature flowing over the flat surface, L (meters) is the length, and As (meters2) is the area. The surface temperature Ts (°C), is assumed to be Ts > T∞. The heat transferred per unit area or heat flux is calculated as: Where q (in Watts) is the heat energy transferred per unit time. While the total heat energy transferred is defined as . Where (W/m C) is the average heat transfer coefficient. Apparatus 1. 1500 Watt hot-air gun 2. Aluminium Disc 3. Rohacell insulation 4. Chromel Alumel thermocouple 5. Digital thermometers 6. Stopwatch. Procedure The arm of the heat gun was familiarized before beginning the experiment. The thermocouple marked air jet was plugged into the digital thermometer and used recording the air temperature. The arm with or holding the air gun was moved away from the block of insulation consisting of the aluminium disc. Then the hot air gun was switched on and given time to warm up, for approximately 5 minutes; at same time, air jet was directed onto the heat proof mat, and the aluminium block was kept away from the hot air jet; and the heater control was adjusted for providing steady air temperature at the range of between 70ºC to 90ºC-the heater maintained at No. 1 and did not exceed 100ºC. When the air exited temperature was constant at the air gun moved back over the block incorporating the aluminium disc, and simultaneously the stopwatch was started. Time, the surface temperature of the disc and the air temperature of the heated jet were recorded. Results Given Variables: Disc Area (A)= 0.004418 m2; Mass(m)=0.07632 Kg; Length scale for the disc, x=0.006 m; T­film=323 K ; k*=0.028 W/mk; µ*= 1.96E-05 kg/ms ; = 1.08 kg/m3 ; v= 1.81448E-05 m2/s; β= 0.003096 K-1 T =22.4 K; Gr=6.632E6; Gr*Pr=4.602E6 For Gr*Pr= 103 109 then C=0.53; n=0.25 Then h=7.893 W/m2C Forced Convection Measurements Experimental Data Data Points Time (seconds) Jet Temperature (°C) Disc Surface Temperature (°C) exp(time) K 1 0 63.5 21.6 1.00 0.028 2 15 62.9 25.8 0.66 R Squared 3 30 62.7 27 0.43 0.56 4 45 62.7 28.1 0.28 5 60 62.5 30.7 0.19 6 80 62.1 32.7 0.11 7 100 60.1 30.5 0.06 8 120 61.9 36.2 0.03 9 150 61.7 38.5 0.01 10 180 61.4 40.6 0.01 11 210 61.3 41.8 0.00 12 250 61.3 43.9 0.00 13 290 61.1 45.5 0.00 14 330 60.9 47.2 0.00 15 390 61.1 48.9 0.00 16 450 61.1 50.1 0.00 17 553 61.2 51.6 0.00 18 560 61.5 52.7 0.00 19 750 61.8 53.4 0.00 Free convection Experimental Data Data Points Time (seconds) Jet Temperature (°C) Disc Surface Temperature (°C) (exp(time))^-0.25 K 1 0 25.3 53.9 1.00 0.028 2 30 24.1 53.1 1.23 R Squared 3 60 23.4 52.4 1.52 0.26 4 90 23.3 51.6 1.88 5 120 23.4 50.8 2.32 6 150 23.4 50.2 2.86 7 180 23.5 49.4 3.53 8 210 23.5 48.1 4.35 9 240 23.5 46.9 5.37 10 300 23.4 45.8 8.17 11 360 23.5 44.7 12.43 12 420 23.4 43.7 18.92 13 480 23.4 43.7 28.79 14 570 23.5 42.3 54.05 15 660 23.3 40.9 101.49 16 750 23.5 39.8 190.57 17 930 23.6 37.6 671.83 18 1110 23.6 35.8 2368.47 19 1290 23.6 34.2 8349.86 20 1470 23.9 32.9 29436.77 21 1650 23.6 31.8 103777.04 22 1980 23.7 30.1 1045493.94 23 2310 23.7 28.8 10532749.91 24 2400 24 28.6 19776402.66 Uncertainity at t=200 seconds Ts= 41.8 C = 61.4 C Tsi = 41.7 C i= 61.35 C UӨ= 3.909430525 Ө=0.01 Uh=3.909430525*0.07632*0.53/0.01*2*0.004418= ± 0.139728 W/m2K Uncertainity at t=1000 Seconds; free convection Ts= 23.6 C = 37.6 C Tsi = 23.5 C i= 39.8 C UӨ= 2.855707957 Ө0.25+1=0 Uh= ± W/m2K Discussion By introducing painting, at 70C the will be radiation of 745.5 Watts, the radiant thermal flux per square meter reflected by the wall. And if the plate was inverted in the free convection there would be no change in coefficient of heat of transfer, since this is dependent on materiel. NUD=21.96 W/mC When calculating the turbulent’s mixing coefficient for the Nusselt number: there was an error associated with the equation. References Ahuja, P. (2009). Chemical engineering thermodynamics. New Delhi: PHI Learning Private Ltd. Bejan, A. (2013). Convection heat transfer, fourth edition. Hoboken, N.J.: Wiley. Çengel, Y., Boles, M. and Kanoğlu, M. (2015). Thermodynamics. Singapore: McGraw-Hill. Dobre, T. and Sanchez Marcano, J. (2007). Chemical engineering. Weinheim: Wiley-VCH. Ghasem, N. (2012). Computer methods in chemical engineering. Boca Raton, FL: CRC Press. Raju, K. (2010). Fluid mechanics, heat transfer, and mass transfer. Hoboken, N.J.: Wiley. Richardson, J., Harker, J. and Backhurst, J. (2007). Chemical engineering. Volume 2, Particle technology and separation processes. Boston: Butterworth-Heinemann. Read More